Blue Chamber Meaning

(idiomatic) A forbidden room.

Example: 1855, Catherine Grace F. Gore, The Heir of Selwood, G. Routledge & Co., page 262:
  â€œ […] Matty would scarcely find fault with my visiting their blue chamber. […] ”
1859 June 18, Charles Dickens, A Tale of Two Cities, Book the Second, Chapter VI, in All the Year Round, Volume I, Number 8, page 172:
  On Sundays, Miss Pross dined at the Doctor’s table, but on other days persisted in taking her meals, at unknown periods, either in the lower regions, or in her own room on the second floor—a blue chamber, to which no one but her Ladybird ever gained admittance.
1925, Sabine Baring-Gould, Cornish characters and strange events
  My mother dared not break the lock, as my father had prohibited any one from entering this, his blue chamber; and what was worse, he had the key.